Front-Side Imager Having a Reduced Dark Current on a SOI Substrate

ABSTRACT

A front-side image sensor may include a substrate in a semiconductor material and an active layer in the semiconductor material. The front side image sensor may also include an array of photodiodes formed in the active layer and an insulating layer between the substrate and the active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/840,164, entitled “Front-Side Imager Having a Reduced Dark Current ona SOI Substrate,” filed on Aug. 31, 2015, which claims priority toFrench application 1460236 filed on Oct. 24, 2014, which applicationsare incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to so-called “front-side” image sensor arraysfabricated in CMOS technology.

BACKGROUND

FIG. 1 is a schematic sectional view of a pixel of a front-side CMOSimage sensor. In this figure and the following, the different pixelelements are shown with dimensions chosen to make the figuresintelligible and are not drawn to scale. The doping levels of the P-typeconductivity zones are shown with shades of gray that are all the darkeras the doping levels are high.

The image sensor is formed in an active layer 10, usually of P-typeconductivity, having a doping level noted P−. The layer 10 is formed ona substrate 12, often of P-type conductivity. The layer 10 may have athickness between 3 and 6 microns, while the substrate may have athickness of 780 microns.

A buried layer 14 of N-type conductivity, close to the upper face of thelayer 10, forms a photodiode with the layer 10. As shown, the portion ofthe layer 10 above the zone 14 may have a higher doping level than thelayer 10 to provide a passivation of the top interface. The upper faceof the layer 10 carries various elements for controlling the pixel,especially a transfer gate TG. These elements and other metal tracks areembedded in a passivation layer 16.

The pixel may be laterally isolated from its neighboring pixels bytrench isolators 18, typically including semiconductor oxide, whichextend throughout the thickness of the active layer 10. Alternatively,the insulation between the pixels may be achieved by an over-doping(P-type) relative to the layer 10, but such insulation is known to beless effective from both an electrical and an optical point of view. Thespacing between the trench isolators 18 defines the size of the pixels.

In the case of a color sensor, color filters 19 are formed on the layer16 in correspondence with the pixels. The filters 19 usually bearindividual collimating lenses 20.

In operation, during an integration phase, the photons absorbed in theactive layer, i.e. the region 10 of the photodiode, generate electronsthat are stored in the region 14 of the photodiode. At the end of theintegration phase, the stored charge is proportional to the amount oflight received by the photodiode throughout the duration of theintegration phase. After the integration phase, the stored charge istransferred through the transfer gate TG to the control elements.

A recurring problem of this pixel structure is the generation ofcarriers in the photodiode in the absence of light, causing a so-calleddark current. The dark current is not the same for all pixels, orbetween two integration phases of the same pixel. This phenomenonproduces a visible noise in the captured images, which is particularlyconspicuous in low light conditions.

The origins of dark current are not well known. An identified source isthe presence of defects or impurities In the semiconductor and thevarious interfaces between the active layer, region 10, and theinsulating materials that surround it. The semiconductor material andthe insulating material are not structurally equivalent, resulting in“construction” defects at the interfaces. All these defects areelectrically active.

The interface defects may be neutralized by degenerating thesemiconductor side of the silicon-insulator interface. Such adegeneration may be produced by over-doping the semiconductor side sothat it has the same properties as a metal, in which thegeneration-recombination phenomena are balanced naturally. A perfectdegeneration is difficult to achieve, whereby defects remain, but insmaller quantities.

In the case of a P-type active region 10, the electrons generated byinterface defects diffuse to the storage region, i.e. region 14. Theseelectrons participate in the dark current of the photodiode. To limitthis phenomenon, interfaces that may present a poor surface state areneutralized, such as the interface between the trench isolators 18 andthe layer 10. As shown, a P-type layer having a higher doping level thanthe active layer 10 may line the trench isolators 18. The doping levelmay be the same, P+, as the substrate. Thus, the generated electronsthat can diffuse to the region 14 are less numerous. Despite thesemeasures, a dark current may still remain an issue in front-side imagesensors.

SUMMARY

In an embodiment, a front-side image sensor comprises a substrate thatincludes a semiconductor material, an active layer that includes asemiconductor material, an array of photodiodes formed in the activelayer, and an insulating layer between the substrate and the activelayer. The insulating layer may be a silicon oxide layer having athickness selected to reflect photons in the visible range. The sensormay further comprise, between the insulating layer and the active layer,an intermediate layer of the same conductivity type as the active layer,having a higher doping level than the active layer.

In operation, the substrate may be biased at a voltage lower than thatof the active layer. The substrate and the insulating layer may be anintegral part of an SOI substrate. The sensor may further include apassivation layer on the active layer, an array of colored filters onthe passivation layer, and an array of collimating lenses on the filterarray.

A method aspect is directed to a method of producing a front-side imagesensor and may include forming an active layer on an SOI substrate,forming a photodiode array in the active layer, and forming an array ofcolor filters and collimating lenses on the active layer. The method mayalso include forming an intermediate layer on the insulating layer, ofthe same conductivity type as the active layer and having a doping levelhigher than the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a pixel of a front-side CMOSimage sensor in accordance with the prior art.

FIG. 2 is a schematic cross-section of an embodiment of a reduced darkcurrent pixel according to the present invention.

FIG. 3 is a schematic cross-section of another embodiment of a low darkcurrent pixel according to an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The inventor has explored the assumption that an electron sourcecontributing to the dark current in a front-side image sensor could alsobe the substrate. Indeed, although the substrate is P-doped, i.e. themajority carriers are positive, electrons generally always remainaccording to the relationship NpNn=ni2, where Np and Nn are the numbersof positive and negative carriers, respectively, and ni is the intrinsicconcentration of the semiconductor material at a given temperature. Theinventor theorizes, without intending to be bound thereto, that thesenegative carriers or electrons could under certain conditions migratefrom the substrate to the active layer, even if the P-doping level ofthe active layer is lower than that of the substrate.

According to this assumption, the contribution of the substrate to thedark current could be reduced or eliminated by electrically isolatingthe active layer from the substrate. The insulation between thesubstrate and the active region may be implemented by forming the imagesensor on a Silicon On Insulator (SOI) substrate.

FIG. 2 illustrates a resulting pixel of an image sensor. This pixel maybe identical in all respects to that of FIG. 1, except that thesubstrate is an SOI substrate comprising a bulk region 22-1 in P-typesilicon, covered by a silicon oxide layer 22-2.

The oxide layer 22-2 may have a thickness between 10 and 200 nm. Byrestricting the thickness range to 100-200 nm, this layer then acts as amirror for photons having a wavelength around the visible spectrum.Incident photons thus reflected to the active layer contribute tocharging the photodiode. This results in an increase of the pixelsensitivity.

FIG. 3 illustrates an alternative embodiment of the pixel of FIG. 2. Thelayer 22-2 is used as a dielectric of a capacitor. The substrate 22-1 isbiased at a voltage V1 lower than the voltage of the active region 10,which is generally grounded. Then the voltage V1 applied is negative. Inthis case, the interface between the active region 10 and the oxideresponds to voltage V1 by the accumulation at the interface of themajority carriers from the active region 10. The voltage induced by thepositive charge accumulation at the interface between the active region10 and the oxide will be designated V2 hereinafter and the voltagedifferential across the capacitor is thus V2−V1.

The voltage V1 to apply may depend on the thickness of the layer 22-2,so in fact on the value of the capacitor. Typically, the differentialV2−V1 may range between 0.2 and 0.4 volt for a thickness of 20 nm, andrange between 1.5 and 3 volts for a thickness of 150 nm.

The layer 24 of FIG. 2 and the capacitor configuration of FIG. 3 may beoptional. These elements may be used to improve the results obtainedthrough the insulating layer 22-2 alone, without applying the biasvoltage V1. The inventor has observed that the insulating layer 22-2used without these options already significantly reduces the effect ofthe substrate on the dark current.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

What is claimed is:
 1. A front-side image sensor comprising: a substratecomprising a semiconductor material; an active layer disposed over thesubstrate; an array of photodiodes in the active layer; and aninsulating layer between the substrate and the active layer, thesubstrate being configured to be biased at a voltage lower than avoltage of the active layer during operation so that an interfacebetween the active layer and the insulating layer responds to thevoltage by an accumulation of a positive charge at the interface.
 2. Thefront-side image sensor of claim 1, wherein the insulating layercomprises a silicon oxide layer having a thickness in a range to reflectphotons in a visible range.
 3. The front-side image sensor of claim 1,further comprising an intermediate layer between the insulating layerand the active layer, the intermediate layer having a same conductivitytype as the active layer and having a higher doping level than theactive layer.
 4. The front-side image sensor of claim 1, wherein thesubstrate is configured to be biased at a voltage lower than a voltageof the active layer during operation.
 5. The front-side image sensor ofclaim 1, wherein the substrate and the insulating layer define asilicon-on-insulator (SOI) substrate.
 6. The front-side image sensor ofclaim 1, further comprising: a passivation layer disposed over theactive layer; an array of colored filters disposed over the passivationlayer; and an array of collimating lenses disposed over the array ofcolored filters.
 7. A method of producing a front-side image sensorcomprising: forming an active layer over a silicon-on-insulator (SOI)substrate; forming an array of photodiodes in the active layer; formingan array of color filters and collimating lenses over the active layer;and forming a trench isolator extending through the active layer to aninsulating layer of the SOI substrate.
 8. The method of claim 7, furthercomprising forming an intermediate layer over the insulating layer, theintermediate layer having a same conductivity type as the active layerand having a doping level higher than the active layer.
 9. The method ofclaim 7, wherein forming the array of photodiodes comprises: forming aburied layer having an opposite doping type than the active layer in theactive layer.
 10. The method of claim 9, further comprising forming atransfer gate over the active layer, the transfer gate being coupled tothe buried layer.
 11. The method of claim 7, wherein the SOI substratecomprises a silicon oxide layer having a thickness in a range to reflectphotons in a visible range.
 12. The method of claim 7, furthercomprising: forming a passivation layer disposed between the activelayer and the array of color filters.
 13. The method of claim 12,further comprising: forming metal tracks embedded in the passivationlayer.
 14. The method of claim 7, further comprising lining the trenchisolator with a higher doping level than the active layer.
 15. Themethod of claim 14, wherein the lining has the same doping type as thesubstrate.
 16. A method of operating a front-side image sensor, themethod comprising: accumulating positive charge at an interface betweenan active layer and an insulating layer by applying a potentialdifference across the insulating layer disposed over a substratecomprising a semiconductor material, wherein the active layer isdisposed over the insulating layer, wherein an array of photodiodes isdisposed in the active layer, wherein the insulating layer is disposedbetween the substrate and the active layer.
 17. The method of claim 16,further comprising providing an intermediate layer over the insulatinglayer, the intermediate layer having a same conductivity type as theactive layer and having a doping level higher than the active layer. 18.The method of claim 16, wherein the array of photodiodes comprises: aburied layer having an opposite doping type than the active layer in theactive layer.
 19. The method of claim 18, further comprising providing atransfer gate over the active layer, the transfer gate being coupled tothe buried layer.
 20. The method of claim 18, further comprising:providing an array of color filters and collimating lenses over theactive layer; providing a passivation layer disposed between the activelayer and the array of color filters; and providing metal tracksembedded in the passivation layer.
 21. The method of claim 18, providinga trench isolator extending to the insulating layer.
 22. The method ofclaim 21, wherein the trench isolator is lined with a higher dopinglevel than the active layer.